With the continuous advancement of the microelectronics technology, intelligent electric devices with microelectronic elements and different functions have emerged and are available in a wide variety, bringing great convenience to people's lives. Generally speaking, the circuits in such electric devices and the electronic elements used therein are designed according to the alternating-current (AC) voltage specification (e.g., 110 or 220 V) of predetermined countries only. In other words, the circuits and electronic elements are operable only in an environment where the corresponding AC voltage specification applies. If an electric device for use in a low-voltage environment is erroneously plugged into an AC power socket of a high-voltage specification, the high-voltage AC power not only can damage the circuits and electronic elements in the electric device, but also can cause injury or even death to the user who is troubleshooting, repairing, or performing maintenance work on the electric device, for the high-voltage AC power flowing into the electric device may leak to the user through the electric device.
The problems stated above rarely happened twenty years ago, when international travel was not as common as today. In the past two decades, however, with the rapid development of the communication, transportation, and travel industries, people have crossed country borders more frequently and at higher speed. It is very likely, therefore, that people are moving between environments of totally different AC voltage specifications without knowing it. For instance, one who is visiting different countries in a short period of time may buy an electric device in a country adopting a low-voltage specification (e.g., 110 V AC power) but does not have a chance to use the electric device until he or she arrives in a country adopting a high-voltage specification (e.g., 220 V AC power). As a result, the high-voltage AC power in the latter country may damage the circuits and components of the electric device immediately, rendering the electric device out of order. Should the user try to check or fix the electric device without knowing the real cause, the high-voltage AC power remaining in the electric device may leak to the user, causing injury if not death.
Therefore, the issue to be addressed by the present invention is to design a structurally simple, durable, and low-cost foolproof circuit which enables an electric device to function properly when plugged into a power socket of a low-voltage specification and which blocks high-voltage AC power (or direct-current (DC) power) from entering the electric device when the electric device is plugged into a power socket of a high-voltage specification. Thus, it can be ensured that the electric device is protected from damage attributable to high-voltage AC or DC power and that high-voltage AC or DC power will not leak to the user through the electric device.